Processes and systems for separating streams to provide a transalkylation feed stream in an aromatics complex

ABSTRACT

A process and system for the production of at least one xylene isomer is provided. The process includes passing a first stream to one side of a split shell fractionation column and a second stream to the other side of the column. The first stream has a higher ratio of methyl to C2+ alkyl-substituted C9 aromatic compounds than the second stream. A bottoms stream from the one side is separated and passed as feed to a transalkylation zone.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/860,563 which was filed on Jul. 31, 2013, the contents of which arehereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to separation of hydrocarbons in an aromaticscomplex and more specifically, to the separation of aromatics compoundsused as feed for transalkylation within an aromatics-processing complexproducing xylene isomers.

BACKGROUND OF THE INVENTION

The xylene isomers are produced in large volumes from petroleum asfeedstocks for a variety of important industrial chemicals. The mostimportant of the xylene isomers is para-xylene, the principal feedstockfor polyester, which continues to enjoy a high growth rate from largebase demand. Ortho-xylene is used to produce phthalic anhydride, whichsupplies high-volume but relatively mature markets. Meta-xylene is usedin lesser but growing volumes for such products as plasticizers, azodyes and wood preservers. Ethylbenzene generally is present in xylenemixtures and is occasionally recovered for styrene production, but isusually considered a less-desirable component of C₈ aromatics.

Among the aromatic hydrocarbons, the overall importance of xylenesrivals that of benzene as a feedstock for industrial chemicals. Xylenesand benzene are produced from petroleum by reforming naphtha but not insufficient volume to meet demand, thus conversion of other hydrocarbonsis necessary to increase the yield of xylenes and benzene. Often tolueneis de-alkylated to produce benzene or selectively disproportionated toyield benzene and C₈ aromatics from which the individual xylene isomersare recovered.

An aromatics complex flow scheme has been disclosed by Meyers in theHandbook of Petroleum Refining Processes, 2d. Edition in 1997 byMcGraw-Hill, and is incorporated herein by reference.

Traditional aromatics complexes send toluene to a transalkylation zoneto generate desirable xylene isomers via transalkylation of the toluenewith A9 components. A9 components are present in both the reformatebottoms and the transalkylation effluent. A9 components may also bepresent to some extent in an isomerization effluent. No effort iscurrently made to separate the A9 components based on their source orparticular structure.

SUMMARY OF THE INVENTION

According to an aspect, a process for producing one or more xylenesincludes passing a first stream comprising xylenes and C9 aromatic at afirst ratio of methyl to C2+ alkyl-substituted C9 aromatic compounds toone side of a split shell fractionation column including a verticalbaffle separating the one side from another side. The process furtherincludes passing a second stream comprising xylenes and C9 aromaticcompounds at a second lower ratio of methyl to C2+ alkyl-substituted C9aromatic compounds than the first ratio to the other side of the splitshell fractionation column. The process includes separating a commonoverhead stream from the split shell fractionation column comprisingxylenes. The process also includes separating a first bottoms streamfrom the one side of the split shell fractionation column and separatinga second bottoms stream from the other side of the split shellfractionation column.

According to an aspect, a process for producing one or more xylenesincludes passing a first stream comprising at least a portion of atransalkylation zone effluent stream including C9 aromatic compounds toa transalkylation effluent side of a split shell fractionation columnincluding a vertical baffle separating the transalkylation effluent sidefrom a reformate side. The process also includes passing a second streamcomprising at least a portion of a reformate stream including C9aromatic compounds to the reformate side of the split shellfractionation column. The process includes separating a common overheadstream from the split shell fractionation column comprising xylenes. Theprocess further includes separating a transalkylation effluent sidebottoms stream comprising C9 aromatic compounds from the transalkylationeffluent side of the split shell fractionation column and separating areformate side bottoms stream comprising C9 aromatic compounds from thereformate side of the split shell fractionation column.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an aromatics complex;

FIG. 2 illustrates an energy-efficient aromatics complex;

FIG. 3 illustrates an aromatics complex in accordance with variousembodiments;

FIG. 4 illustrates an energy-efficient aromatics complex in accordancewith various embodiments.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles ofexemplary aspects. The scope of the invention should be determined withreference to the claims.

In current aromatics complexes, different sources of A9 components arecombined and passed to the transalkylation zone without regard to thealkyl groups of the A9 species and no effort is made to separate the A9species based on the alkyl groups. It has been identified thatmethyl-substituted A9 species used as feed to the transalkylation zoneincreases the yield of desirable A8 components since Ethyl- and higheralkyl groups are cracked from the aromatics rings to form less valuablebenzene and fuel gas. In this regard, an apparatus and process areprovided for producing one or more xylenes that includes separating ormaintaining as separate streams having higher amounts ofmethyl-substituted C9 aromatic compounds from streams having greateramounts of C2+ ethyl-substituted aromatic compounds. The process andapparatus utilize a split-shell fractionation column so that thesestreams may be separated without significantly increasing capital orenergy requirements. Advantageously, when the aromatics complex iscombined with dedicated gasoline production, the C9 aromatic compoundswith C2+ alkyl groups can be directed to the gasoline pool withoutfurther processing, limiting the amount of byproduct Benzene and fuelgas while increasing the production of para-xylene.

The feedstream to the present process generally comprises alkylaromatichydrocarbons of the general formula C₆H(_(6-n))R_(n), where n is aninteger from 0 to 5 and each R may be CH₃, C₂H₅, C₃H₇, or C₄H₉, in anycombination. The aromatics-rich feed stream to the process of theinvention may be derived from a variety of sources, including withoutlimitation catalytic reforming, steam pyrolysis of naphtha, distillatesor other hydrocarbons to yield light olefins and heavier aromatics-richbyproducts (including gasoline-range material often referred to as“pygas”), and catalytic or thermal cracking of distillates and heavyoils to yield products in the gasoline range. Products from pyrolysis orother cracking operations generally will be hydrotreated according toprocesses well known in the industry before being charged to the complexin order to remove sulfur, olefins and other compounds which wouldaffect product quality and/or damage catalysts or adsorbents employedtherein. Light cycle oil from catalytic cracking also may bebeneficially hydrotreated and/or hydrocracked according to knowntechnology to yield products in the gasoline range; the hydrotreatingpreferably also includes catalytic reforming to yield the aromatics-richfeed stream. If the feed stream is catalytic reformate, the reformerpreferably is operated at high severity to achieve high aromatics yieldwith a low concentration of nonaromatics in the product.

FIG. 1 is a simplified flow diagram of an exemplary aromatics-processingcomplex of the known art directed to the production of at least onexylene isomer. The complex may process an aromatics-rich feed which hasbeen derived, for example, from catalytic reforming in a reforming zone6. The reforming zone generally includes a reforming unit 4 thatreceives a feed via conduit 2. The reforming unit will typicallycomprises a reforming catalyst. Usually such a stream will also betreated to remove olefinic compounds and light ends, e.g., butanes andlighter hydrocarbons and preferably pentanes; such removal, however, isnot essential to the practice of the broad aspects of this invention andis not shown. The aromatics-containing feed stream contains benzene,toluene and C₈ aromatics and typically contains higher aromatics andaliphatic hydrocarbons including naphthenes.

The feed stream is passed via conduit 10 via a heat exchanger 12 toreformate splitter 14 and distilled to separate a stream comprising C₈and heavier aromatics, withdrawn as a bottoms stream via a bottomsoutlet 15 in conduit 16, from toluene and lighter hydrocarbons recoveredoverhead via conduit 18. The toluene and lighter hydrocarbons are sentto extractive distillation process unit 20 which separates a largelyaliphatic raffinate in conduit 21 from a benzene-toluene aromaticsstream in conduit 22. The aromatics stream in conduit 22 is separated,along with stripped transalkylation product in conduit 45 and overheadfrom para-xylene finishing column in conduit 57, in benzene column 23into a benzene stream in conduit 24 and a toluene-and-heavier aromaticsstream in conduit 25 which is sent to a toluene column 26. Toluene isrecovered overhead from this column in conduit 27 and may be sentpartially or totally to a transalkylation unit 40 as shown and discussedhereinafter.

A bottoms stream from the toluene column 26 is passed via conduit 28,along with bottoms from the reformate splitter in conduit 16, aftertreating via clay treater 17, and recycle C₈ aromatics in conduit 65, tofractionator 30. The fractionator 30 separates concentrated C₈ aromaticsas overhead in conduit 31 from a high-boiling stream comprising C₉, C₁₀and heavier aromatics as a bottoms stream in conduit 32. This bottomsstream is passed in conduit 32 to heavies column 70. The heavy-aromaticscolumn provides an overhead stream in conduit 71 containing C₉ and atleast some of the C₁₀ aromatics, with higher boiling compounds,primarily C₁₁ and higher alkylaromatics, being withdrawn as a bottomsstream via conduit 72.

The C₉+ aromatics from heavies column in conduit 71 is combined with thetoluene-containing overhead contained in conduit 27 as feed totransalkylation reactor 40, which contains a transalkylation catalyst asknown in the art to produce a transalkylation product comprising benzenethrough C₁₁+ aromatics with xylenes as the focus. The transalkylationproduct in conduit 41 is stripped in stripper 42 to remove gases inconduit 43 and C₆ and lighter hydrocarbons which are returned viaconduit 44 to extractive distillation 20 for recovery of light aromaticsand purification of benzene. Bottoms from the stripper are sent inconduit 45 to benzene column 23 to recover benzene product andunconverted toluene.

The C₈-aromatics overhead provided by fractionator 30 containspara-xylene, meta-xylene, ortho-xylene and ethylbenzene and passes viaconduit 31 to para-xylene separation process 50. The separation processoperates, preferably via adsorption employing a desorbent, to provide amixture of para-xylene and desorbent via conduit 51 to extract column52, which separates para-xylene via conduit 53 from returned desorbentin conduit 54; the para-xylene is purified in finishing column 55,yielding a para-xylene product via conduit 56 and light material whichis returned to benzene column 23 via conduit 57. A non-equilibriummixture of C₈-aromatics raffinate and desorbent from separation process50 is sent via conduit 58 to raffinate column 59, which separates araffinate for isomerization in conduit 60 from returned desorbent inconduit 61.

The raffinate, comprising a non-equilibrium mixture of xylene isomersand ethylbenzene, is sent via conduit 60 to isomerization reactor 62.The raffinate is isomerized in reactor 62, which contains anisomerization catalyst to provide a product approaching equilibriumconcentrations of C₈-aromatic isomers. The product is passed via conduit63 to deheptanizer 64, which removes C₇ and lighter hydrocarbons withbottoms passing via conduit 65 to xylene column 30 to separate C₉ andheavier materials from the isomerized C₈-aromatics. Overhead liquid fromdeheptanizer 64 is sent to stripper 66, which removes light materialsoverhead in conduit 67 from C₆ and C₇ materials which are sent viaconduit 68 to the extractive distillation unit 20 for recovery ofbenzene and toluene values.

There are many possible variations of this scheme within the known art,as the skilled routineer will recognize. For example, the entire C₆-C₈reformate or only the benzene-containing portion may be subjected toextraction. Para-xylene may be recovered from a C₈-aromatic mixture bycrystallization rather than adsorption. Meta-xylene as well aspara-xylene may be recovered from a C₈-aromatic mixture by adsorption,and ortho-xylene may be recovered by fractionation. Alternatively, theC₉- and heavier stream or the heavy-aromatics stream is processed usingsolvent extraction or solvent distillation with a polar solvent orstripping with steam or other media to separate highly condensedaromatics as a residual stream from C₉+ recycle to transalkylation. Insome cases, the entire heavy-aromatic stream may be processed directlyin the transalkylation unit. The present invention is useful in theseand other variants of an aromatics-processing scheme, aspects of whichare described in U.S. Pat. No. 6,740,788 which is incorporated herein byreference.

Referring to FIG. 2, another exemplary aromatics complex withmodifications to improve energy efficiency is illustrated. Theenergy-efficient aromatics complex is described in U.S. PatentPublication No. 2012/0048720, which is incorporated by reference herein,in its entirety. For ease of reference, a parallel numbering system isemployed to those of FIGS. 1 and 2 and similar elements will not bedescribed herein in detail. It should be noted that several variationsof the process flow and equipment in this complex are possible, andcontemplated herein. The energy efficient aromatics complex includesfirst and second xylene columns 130 and 133. In this example, firstxylene column 130 is a low-pressure column while second xylene column isa high-pressure column. In the reforming zone 106, the feed stream ispassed via conduit 102 to reforming unit 104 which includes a reformingcatalyst, as is known in the art and described above with regard toFIG. 1. The reformate is passed via conduit 110 via heat exchangers 112and 113, which raise the temperature of the feed stream, to reformatesplitter 114. The heat exchange may supplied via conduits 212 and 213respectively from the net para-xylene product and the recoveredpara-xylene separation process recovered desorbent as discussed later inthis section.

As in FIG. 1, C₈ and heavier aromatics are withdrawn as a bottoms streamvia bottoms outlet in conduit 116 while toluene and lighter hydrocarbonsrecovered overhead via conduit 118 are sent to extractive distillationprocess unit 120 which separates a largely aliphatic raffinate inconduit 121 from a benzene-toluene aromatics stream in conduit 122. Thearomatics stream in conduit 122 is separated, along with strippedtransalkylation product in conduit 144 and overhead from para-xylenefinishing column in conduit 157, in fractionator 123 into a benzenestream in conduit 124 and a toluene-and-heavier aromatics stream inconduit 125 which is sent to a toluene column 126. Toluene is recoveredoverhead from this column in conduit 127 and may be sent partially ortotally to a transalkylation unit 140 as shown and discussedhereinafter.

A bottoms stream from the toluene column 126 is passed via conduit 128,along with bottoms from the reformate splitter in conduit 116, aftertreating via clay treater 117, and recycle C₈ aromatics in conduit 138,to low-pressure xylene column 130. Other C8-aromatics streams havingsignificant contents of C9 and heavier aromatics, including streamsobtained from sources outside the complex, also may be processed in thiscolumn; a portion of deheptanizer bottoms in stream 165 also may beincluded depending on overall energy balances. The low-pressure xylenecolumn separates concentrated C₈ aromatics as overhead in conduit 131from a high-boiling stream comprising C₉, C₁₀ and heavier aromatics as abottoms stream in conduit 132.

Simultaneously, an isomerized C₈-aromatics stream is passed via conduit165 to a high-pressure second xylene column 133. This is characterizedas a lower-boiling feed stream which contains a lower concentration ofheavy materials subject to decomposition than the feed to column 130,and the column pressure thus can be increased in order to effect energysavings. Other C₈-aromatics-containing streams having similarly lowcontents of C₉-and-heavier aromatics, including streams obtained fromsources outside the complex, also may be contained in the feed stream tothis column. The second xylene column separates a second C₈-aromaticsstream as overhead in conduit 134 from a second C₉-and-heavier stream inconduit 139. At least a portion of overhead vapor from the high-pressurexylene column in conduit 134 preferably is employed to reboillow-pressure xylene column 130 in reboiler 135, leaving as a condensedliquid to the xylene-separation process 150 in conduit 136 as well asreflux (not shown) to column 133. In addition, the overhead in conduit134 preferably is used to provide energy to the reboiler of extractcolumn 152 as well as other such services which are described later orwill be apparent to the skilled routineer.

The C₉+ bottoms stream passing to reboiler 137 may provide energy viaone or both of the stream before the reboiler in conduit 270 and theheated stream from the reboiler in conduit 259 for reboilingrespectively one or both of heavy-aromatics column 170 and raffinatecolumn 159; the bottoms stream after heat exchange would be sent to theheavy-aromatics column 170. Other similar heat-exchange services will beapparent to the skilled routineer. The net bottoms stream in conduit 138usually is passed through column 130 or may be in conduit 139 combineddirectly with the stream in conduit 132 to heavies column 170. Theheavies column provides an overhead a stream in conduit 171 containingC₉ and at least some of the C₁₀ aromatics, with higher boilingcompounds, primarily C₁₁ and higher alkylaromatics, being withdrawn as abottoms stream via conduit 172. This column may be reboiled by xylenecolumn bottoms in conduit 270, as discussed above. Overhead vapor fromcolumns 130 and 170 also may generate steam respectively via conduits230 and 271 as indicated, with condensed liquids either serving asreflux to each column or as net overhead respectively in streams 131 or171.

The C₉+ aromatics from heavies column in conduit 171 is combined withthe toluene-containing overhead contained in conduit 127 as feed totransalkylation reactor 140 to produce a transalkylation productcontaining xylenes. The transalkylation product in conduit 141 isstripped in stripper 142 to remove gases in conduit 143 and C₇ andlighter liquids which are returned via conduit 144 to extractivedistillation 120 for recovery of light aromatics following stabilizationin isomerate stripper 166. Bottoms from the stripper are sent in conduit145 to benzene column 123 to recover benzene product and unconvertedtoluene.

The first and second C₈-aromatics streams provided by xylene columns 130and 133, containing para-xylene, meta-xylene, ortho-xylene andethylbenzene, pass via conduit 131 and 136 to xylene-isomer separationprocess 150. The description herein may be applicable to the recovery ofone or more xylene isomers other than para-xylene; however, thedescription is presented for para-xylene for ease of understanding. Theseparation process operates via a moving-bed adsorption process toprovide a first mixture of para-xylene and desorbent via conduit 151 toextract column 152, which separates para-xylene via conduit 153 fromreturned desorbent in conduit 154. Extract column 152 preferably isoperated at an elevated pressure, at least about 300 kPa and morepreferably about 500 kPa or higher, such that the overhead from thecolumn is at sufficient temperature to reboil finishing column 155 viaconduit 256 or deheptanizer 164 via conduit 265. Heat supplied forreboiling duty via conduits 256 and 265 results in the condensation ofthe extract in these streams which is either or both refluxed to column152 (not shown) or sent as a net stream in conduit 153 to finishingcolumn 155. The para-xylene is purified in finishing column 155,yielding a para-xylene product via conduit 156 and light material whichis returned to benzene column 123 via conduit 157.

A second mixture of raffinate, as a non-equilibrium blend of C₈aromatics, and desorbent from separation process 150 is sent via conduit158 to raffinate column 159, which separates a raffinate toisomerization in conduit 160 from returned desorbent in conduit 161. Theraffinate column may be operated at higher pressure to generate steamvia conduit 260 or to exchange heat in other areas of the complex;condensed liquids from such heat exchange either serve as reflux to theraffinate column or as net overhead in conduit 160. Recovered desorbentin conduits 154 and 161 and net finishing column bottoms may heat theincoming feed stream in conduit 110 via conduits 213 and 212,respectively. In an energy-efficient aromatics complex, the firstfractionation column may be operated at a low pressure to separate afirst C₈-aromatics stream from a first C₉-and-heavier aromatics stream,the second fractionation column may be operated at an elevated pressureto separate a second C₈-aromatics stream from a second C₉-and-heavieraromatics stream. In this regard, an overhead stream from the secondcolumn may be circulated to provide heat to a reboiler of the firstcolumn. The low pressure typically is between 100 and 800 kPa and theelevated pressure is chosen to enable heat transfer from the firstcolumn to the second and typically is at least about 400 kPa above thelow pressure.

The stream to the second column may contains less than about 10 weight-%C₉+ aromatics, more often less than about 5 weight-% C₉+ aromatics, andfrequently less than about 2 weight-% C₉+ aromatics. The complex mayalso allow operating the second fractionation column at a pressure thatwould enable the overhead to provide heat to generate steam useful in anassociated processing complex. Further, the C₈-aromatics fractionatormay comprise three or more columns comprising additional heat exchangebetween overheads and reboilers in an analogous manner to the abovedescription.

Turning now to FIG. 3, an aromatics complex and process in accordancewith one aspect will be illustrated and described. As shown in FIG. 3,according to this aspect, the xylene fractionation column includes asplit shell fractionation column 330. It includes a baffle 333 extendingfrom the bottom of the column dividing the tray section of thefractionation column into two sides. The baffle extends to a height lessthan the full height of the split shell fractionation column so that acommon overhead may be collected from above the baffle. A first streamenters the split shell fractionation column 330 on a first side 305 ofthe baffle 333. A second stream enters the column 330 on a second side310 of the baffle 333. By one approach, the first stream has a highermole ratio of methyl- to C2+ alkyl-substituted C9 and C10 aromaticcompounds than the second stream and therefore a higher methyl- tophenyl-ratio. By one approach, the first stream has a ratio of methyl tophenyl groups of between about 2.0 and about 3.0, between about 1.5 andabout 3.5 in another example, and between about 1.0 and 4.0 in anotherexample. On the other hand, by one example, the second stream has aratio of methyl to phenyl groups of between about 1.5 and about 2.5,between about 1.0 and about 3.0 in another example, and between about0.5 and 3.5 in another example.

In the example illustrated in FIG. 3, the first stream includes aportion of a transalkylation zone effluent. The transalkylation zoneeffluent may be treated and or separated such as by fractionation priorto entering the split shell fractionation column 330. In one approach,the first stream includes at least a portion of a bottoms stream fromthe toluene column 26, which is passed to the inlet at the first side305 of the split shell fractionation column via line 28. The secondstream may include a reformate bottoms portion, such as the bottomsstream from reformate splitter 14 passed via line 16. The reformatebottoms stream may be treated, such as by clay treater 17 beforeentering the second side 310 of the split shell fractionation column330. It has been identified that the toluene column bottoms stream has ahigher ratio of methyl to C2+ alkyl-substituted C9 aromatic compoundsthan the reformate bottoms stream. The baffle 333 extends above thelevel of both feed inlets 315 and 320 and a normal operation liquidlevel of the split shell fractionation column 330 to restrict liquidfrom spilling over the baffle 333 and mixing.

The common overhead from the split shell fractionation column 330comprising mixed xylenes is sent via conduit 31 to the para-xyleneisomer separation zone to separate para-xylene from the other xyleneisomers and ethylbenzene as described above with regard to FIG. 1.

Because the baffle 333 extends from the bottom of the split shellfractionation column 330 to a height above the normal operation liquidlevel of the column, the liquid bottoms from the first (transalkylationeffluent) side 305 and the second (reformate) side 310 remain separated.The bottoms from the transalkylation effluent side 305 of the splitshell fractionation column 330 are withdrawn as a first xylene columnbottoms stream through outlet 338, which is passed via conduit 71 to thetransalkylation zone 340 to the transalkylation reactor 40. Asillustrated in FIG. 3, the transalkylation side bottoms stream is sentto the inlet 341 of transalkylation reactor 40. The transalkylationeffluent side bottoms stream may be further processed prior to beingpassed to the transalkylation zone 340. For example, as illustrated inFIG. 3, the transalkylation side bottoms stream may be passed to theheavy aromatic hydrocarbon fractionation column 70 through conduit 331.In this regard, C9 and C10 aromatic compounds may be removed as anoverhead stream from the fractionation column 70 and passed to thetransalkylation reactor 40. Heavier compounds, including C11+ aromaticcompounds may be withdrawn as a bottoms stream via conduit 72. The heavyaromatic fractionation column bottoms stream may be sent to anotherlocation, such as, for example, blending with a gasoline pool where thearomatics complex is integrated with a refinery. Another portion of thetransalkylation effluent side bottoms stream may be passed via conduit335 to reboiler 336 and back to the first side 305 of the split shellfractionation column 330.

By one aspect, a bottoms stream from the reformate side 310 of the splitshell fractionation column 330 is withdrawn through reformate sidebottoms outlet 339 and passed to a destination other than thetransalkylation zone 340. In one approach, illustrated in FIG. 3, wherethe aromatics complex is integrated with a larger refinery, thereformate side bottoms stream may be passed via line 332 to a gasolinepool or other hydrocarbon stream of the refinery or petrochemical plant.Because the bottoms stream will include primarily C9+ aromaticcompounds, it is particularly well suited for blending into the gasolinepool. As illustrated in FIG. 3, another portion of the reformate sidebottoms stream may be passed via conduit 337 to reboiler 334 and back tothe reformate side 310 of the split shell fractionation column 330.

As illustrated in FIG. 3, an A8+ stream including isomerized xylenes isprocessed in isomerization reactor 62 and separated in deheptanizer 64.It has been identified that C9 aromatics in the deheptanizer bottomsstream from the isomerization zone 362 typically have a higher ratio ofmethyl to C2+ alkyl-substituted C9 aromatic compounds than the reformatebottoms stream, similar to the transalkylation zone effluent. By oneaspect, at least a portion of the first stream may include at least aportion of the deheptanizer bottoms stream. In this regard, thedeheptanizer bottoms stream may be withdrawn through outlet 364 andpassed via conduit 65 to the first (transalkylation) side 305 of thesplit shell fractionation column. The stream may be introduced to thefirst side 305 of the split shell fractionation column, in addition oralternatively to the transalkylation zone effluent stream describedpreviously. In the case where both streams are passed to the first side305 of the xylene fractionation column 330, they may be passedseparately into the first side 305 via separate inlets, or combined andintroduced together via a common inlet 315 as illustrated in FIG. 3.Regardless of how the isomerization zone bottoms stream is introducedinto the split shell fractionation column 330, it should be noted thatthe inlet should be positioned below the top of the baffle 333, torestrict liquid spillover and mixing as described previously.

Turning now to FIG. 4, an energy-efficient aromatics complex and processincorporating a split shell fractionation column in accordance with anaspect is illustrated. As shown in FIG. 4, according to this aspect, thefirst xylene fractionation column includes a split shell fractionationcolumn 430. The split shell fractionation column includes a baffle 433extending from the bottom of the column dividing the tray section of thefractionation column 430 into two sides. The baffle extends to a heightless than the full height of the split shell fractionation column sothat a common overhead may be collected from above the baffle. A firststream enters the split shell fractionation column 430 on a first side405 of the baffle 433. A second stream enters the column 430 on a secondside 410 of the baffle 433. By one approach, the first stream has ahigher ratio of methyl- to C2+ alkyl-substituted C9 aromatic compoundsthan the second stream and therefore a higher methyl- to phenyl-ratio.By one approach, the first stream has a ratio of methyl to phenyl groupsof between about 2.0 and about 3.0, between about 1.5 and about 3.5 inanother example, and between about 1.0 and 4.0 in another example. Onthe other hand, by one example, the second stream has a ratio of methylto phenyl groups of between about 1.5 and about 2.5, between about 1.0and about 3.0 in another example, and between about 0.5 and 3.5 inanother example.

In the example illustrated in FIG. 4, the first stream includes aportion of a transalkylation zone effluent. The transalkylation zoneeffluent may be treated and or separated such as by fractionation priorto entering the split shell fractionation column 430. In one approach,the first stream includes at least a portion of a bottoms stream fromthe toluene column 126, which is passed to the inlet 415 at the firstside 405 of the split shell fractionation column 430 via line 128. Thesecond stream may include a reformate bottoms portion, such as thebottoms stream from reformate splitter 114 passed via line 116. Thereformate bottoms stream may be treated, such as by clay treater 117before entering the second side 410 of the split shell fractionationcolumn 430. It has been identified that the toluene column bottomsstream has a higher ratio of methyl to C2+ alkyl-substituted C9 aromaticcompounds than the reformate splitter bottoms stream. The baffle 433extends above the level of both feed inlets 415 and 420 and a normaloperation liquid level of the split shell fractionation column 430 torestrict liquid from spilling over the baffle 433 and mixing.

The common overhead from the split shell fractionation column 430including mixed xylenes is sent via conduit 131 to the para-xyleneisomer separation unit 150 to separate para-xylene from the other xyleneisomers and ethylbenzene as described above with regard to FIG. 2.

Because the baffle 433 extends from the bottom of the split shellfractionation column 430 to a height above the normal operation liquidlevel of the column, the liquid bottoms from the first (ortransalkylation effluent) side 405 and the second (or reformate) side410 remain separated. The bottoms from the transalkylation effluent side405 of the split shell fractionation column 430 are withdrawn as atransalkylation effluent side bottoms stream through outlet 439 and atleast a portion thereof is passed to the transalkylation reactor 140 viainlet 441. The transalkylation side bottoms stream may be furtherprocessed or separated prior to being passed to the transalkylation zone440. For example, as illustrated in FIG. 4, the transalkylation sidebottoms stream may be passed to the heavy aromatic hydrocarbonfractionation column 170 through inlet 471. In this regard, C9 and C10aromatic compounds are removed as an overhead stream from thefractionation column 170 via outlet 472 and passed to thetransalkylation reactor 140. Heavier compounds, including C11+ aromaticcompounds may be withdrawn as a bottoms stream via conduit 172. Theheavy aromatic fractionation column bottoms stream may be sent toanother location, such as, for example, blending with a gasoline poolwhen the aromatics complex is integrated in a refinery. Another portionof the first xylene column bottoms stream may be passed via conduit 435to reboiler 135 and back to the first side 405 of the split shellfractionation column 430.

By one aspect, the bottoms from the reformate side 410 of the splitshell fractionation column 430 is passed to a destination other than thetransalkylation zone 440. In one approach, illustrated in FIG. 4, wherethe aromatics complex is part of a larger refinery, the reformate sidebottoms stream may be passed via line 436 to a gasoline pool or otherhydrocarbon stream of the refinery or petrochemical plant. Because thebottoms stream will include primarily C9+ aromatic compounds, it isparticularly well suited for blending into the gasoline pool. While notillustrated, a portion of the reformate side bottoms stream may beseparated and passed to a reboiler and back to the reformate side 410 ofthe split shell fractionation column 430.

As illustrated in FIG. 4, an A8+ stream including isomerized xylenes isprocessed in isomerization reactor 162 and separated in deheptanizer164. It has been identified that C9 aromatics in the deheptanizerbottoms stream from the isomerization zone typically have a higher ratioof methyl to C2+ alkyl-substituted C9 aromatic compounds than thereformate bottoms stream. In this regard, at least a portion of thedeheptanizer bottoms stream may be passed to the first side 405 of thesplit shell fractionation column 430. In the energy-efficient complexillustrated in FIG. 4, the deheptanizer bottoms stream including C8+aromatics may be withdrawn through outlet 464 and sent to a secondxylene fractionation column 133 via conduit 165 and subsequently to thefirst side 405 of the split shell fractionation column. An overheadstream including C8 aromatics may be withdrawn and passed through line134 as described previously.

A bottoms stream including C9+ aromatic hydrocarbons may be withdrawnfrom the column 133 via outlet 465. In one approach, at least a portionof the stream is passed via conduit 138 and introduced to the splitshell fractionation column 430. The bottoms stream from column 133typically has a relatively lower concentration of C9 aromatichydrocarbons than the transalkylation stream described previously. Inthis regard, by one approach, the bottoms stream from column 133 isintroduced on the first side 405 of the split shell fractionation column430 in addition or alternatively to the transalkylation zone effluentstream described previously. In another approach, the bottoms streamfrom column 133 is introduced on the first side 410 of the split shellfractionation column 430 in addition or alternatively to the reformatestream described previously. The bottoms stream from column 133 may beintroduced below the upper portion of the baffle 433 as with thetransalkylation zone effluent stream and reformate stream or above theupper portion of the baffle 433 in the common overhead region. When thebottoms stream is passed to one of the first side of the xylenefractionation column and the second side of the column, it may be passedseparately into the column via separate inlets, or combined andintroduced with the other stream via a common inlet, an example of whichis illustrated in FIG. 4. In another approach, at least a portion of thesecond xylene fractionation column bottoms stream may be removed andpassed via conduit 437 to another location, such as, for example,blending with a gasoline pool when the aromatics complex is integratedin a refinery.

While the invention herein disclosed has been described by means ofspecific embodiments, examples and applications thereof, numerousmodifications and variations could be made thereto by those skilled inthe art without departing from the scope of the invention set forth inthe claims.

The invention claimed is:
 1. A process for producing one or more xylenesand a C9+ gasoline blending stream in an aromatics complex, comprising:passing a first stream comprising xylenes and C9+ aromatics to a firstside of a split shell fractionation column including a vertical baffleseparating the first side from a second side, wherein the C9+ aromaticsin the first stream have a first ratio of methyl- to phenyl-groups;passing a second stream comprising xylenes and C9+ aromatics to thesecond side of the split shell fractionation column, wherein the C9+aromatics in the second stream have a second ratio of methyl- tophenyl-groups and wherein the first ratio is greater than the secondratio; separating a common overhead stream from the split shellfractionation column comprising xylenes; separating a first bottomsstream from the first side of the split shell fractionation column; andseparating a second bottoms stream from the second side of the splitshell fractionation column; passing at least a portion of the firstbottoms stream as a feed to a transalkylation zone; and passing at leasta portion of the second bottoms stream to a gasoline pool, wherein theat least the portion of the second bottoms stream comprises the C9+gasoline blending stream.
 2. The process according to claim 1, whereinat least a portion of the first stream comprises at least a portion of atransalkylation zone effluent.
 3. The process according to claim 1,wherein at least a portion of the second stream comprises at least aportion of a reformate stream.
 4. The process according to claim 1,wherein at least a portion of the first stream comprises at least aportion of an isomerization zone effluent.
 5. The process according toclaim 4, wherein the portion of the isomerization zone effluent ispassed to a xylene fractionation column and the at least portion of thefirst stream comprises at least a portion of a bottoms stream from thexylene fractionation column.
 6. The process according to claim 1,wherein the first ratio of methyl- to phenyl-groups of the first streamis between about 1.0 and about 4.0.
 7. The process according to claim 1,wherein the second ratio of methyl- to phenyl-groups of the secondstream is between about 0.5 and about 3.5.